An element for a silver salt diffusion transfer process comprises, in general, a light-sensitive material comprising a support having thereon light-sensitive silver halide emulsion layers, an image receiving material comprising a support having thereon an image receiving layer containing physical development nuclei, and a processing solution containing a silver halide solvent.
The principle of the silver salt diffusion transfer process is that silver halide present in exposed areas of an exposed light-sensitive layer is developed with a developing agent present either in a processing solution or in the light-sensitive material. At the same time, silver halide present in the unexposed areas reacts with a silver halide solvent contained in the processing solution to form a soluble silver complex salt, and the resulting silver complex salt diffuses into an image receiving layer and is deposited on physical development nuclei present in the image receiving layer to form a silver image.
The silver salt diffusion transfer process described above is applicable to two element embodiments. One is the "monosheet" type in which a light-sensitive layer and an image receiving layer are provided on the same support, and another is the "strip off" type in which a light-sensitive layer and an image receiving layer are provided on separate support. When using the strip off element in a diffusion transfer process, the light-sensitive material and image receiving material are dipped in a processing solution and then they are brought into contact with each other and pressure is applied by means of rollers to effect the diffusion transfer of silver salt image, after which the image receiving material is stripped off from the light-sensitive material. The present invention relates particularly to the latter type of embodiments.
The silver salt diffusion transfer process is employed for reproducing documents, such as technical drawings, printed matter and handwriting, or producing block copies in the photographic mechanical processes. Accordingly, faithful image reproduction of original manuscripts is required in practicing the silver salt diffusion transfer process.
In the case of embodiments of the "strip off" type, it is necessary for a light-sensitive layer to be brought into perfect face-to-face contact with an image receiving layer of an image receiving material in order to achieve uniform diffusion of the silver complex salt. In general, after a light-sensitive material and an image receiving material are dipped in a processing solution, one material is superposed on another material with the supports on the outside and passed through squeegee rollers, as described in U.S. Pat. No. 4,345,019. These steps are all carried out in a development processing apparatus having processing trough and squeegee rollers. In such a process, bubbles in the processing solution often adhere to the surface of the image receiving layer or the light-sensitive layer, and they remain between the light-sensitive layer and the image receiving layer without being completely removed by passing through pressure applying rollers. Poor adhesion or defective permeation of the processing solution occurs in the spots where the bubbles remain to disturb the diffusion transfer of the silver complex salt, and in result, poor transfer areas in the form of fine spots are generated on the resulting image.
In addition, one of the superposed materials can slide across another material upon the slightest touch after passage through the squeegee rollers due to poor adhesiveness between the light-sensitive layer and the image receiving layer, resulting in distortion of the image to be transferred into the image receiving layer (that is, "shear in transfer"). Alternatively, if adhesion between the light-sensitive layer and the image receiving layer is too strong, it is difficult to strip the image receiving material off the light-sensitive material, or the transferred image is damaged by the strip off.
Further, the image receiving material is generally washed for about 5 seconds or longer after the strip off step because developer and other ingredients used for processing remain in the image receiving material stripped off after the processing, and they will cause stains if the material is allowed to stand as it is. However, if a high molecular substance facilitating stripping off, such as carboxymethyl cellulose or the like, is incorporated in the image receiving layer, in the above-described washing step and the subsequent drying step effusion of the high molecular substance from the image receiving layer causes marks shaped like trickle and spots in transferred image areas of the image receiving layer, and color of silver present at the surface of the transferred image is changed partially. Such marks are quite undesirable from the viewpoint of good appearance of transferred image. Particularly when the transferred image serves as a material for block copy in photographing with a camera, and a plate is made using an electrophotomechanical process, such marks can cause a serious problem of unevenness in printing.
The various problems described above constitute the main factors causing the marked deterioration of reproducibility of diffusion transfer images, such as characters and fine lines, and in reproducibility in conversion of continuous tone images into dot images using a contact screen. Therefore, it is strongly desired to eliminate such defects.
The combined use of gelatin and a high molecular substance facilitating strip off in an image receiving layer for the diffusion transfer process is known. Also, saponin and an anionic surface active agent, e.g., an alkylbenzenesulfonic acid, are used as a coating aid. According to our studies, however, it has been found that the problems described above, for instance, generation of poor transfer areas shaped like spots, facility in stripping a light-sensitive material off an image receiving material, generation of marks and so on, are attributable mainly to the image receiving material, in the strict sense, due to an interaction between a binder in the image receiving layer and a surface active agent. This interaction is observed in the microscopic phase separation of a high molecular substance facilitating strip off produced in gelatin binder (by, e.g., increasing the concentration of the high molecular substance in the binder), and in phenomena attended by elution of the high molecular substance facilitating strip off from the surface of the image receiving layer upon washing with water.